1. Field of the Invention
The present invention relates to filters, and more specifically to a method and apparatus for implementing desired impedance between two nodes or ends that are connected to a non-fixed voltage level.
2. Related Art
Filters are generally implemented to perform corresponding transfer functions as is well known in the relevant arts. A transfer function is determined by, among other aspects, the impedances used to implement the corresponding filter as is also well known in the relevant arts.
Accordingly, it is generally desirable that components be implemented to attain a desired impedance (within an acceptable tolerance limit) between the two ends (xe2x80x9cterminating endsxe2x80x9d) of the impedance. However, due to reasons such as manufacturing process imperfections, it is often difficult to implement components with the desired impedances.
In a prior approach known as trimming, a large impedance substantially equaling the desired impedance, may be used in conjunction with several incremental impedances. Switches may be used to selectively activate only some of the incremental impedances to attain a desired impedance across the terminating ends.
It may be desireable to provide the ability to trim impedances having terminating ends, wherein the terminating ends couple to a non-fixed voltage level while meeting several other requirements (such as ensuring that the implemented component continues to operate within an acceptable impedance range in several operation situations). Such ability would generally increase the design choices for a designer implementing a solution.
Accordingly, what is needed is a method and apparatus which enables an impedance (or the impedance of a component) to be trimmed when neither terminating end is connected to ground.
The present invention allows a desired impedance, which does not change substantially in different operating situations, to be implemented between two terminating nodes not connected to the ground. The desired impedance may be obtained by using a T-network between the two nodes.
The T-network may contain three impedances, with the first two impedances being connected in series between the two terminating nodes. One terminal of the third impedance may be connected to a fixed voltage (such as ground), and the other terminal may be connected to the junction of the first two impedances. Assuming the three impedances are represented by ZA, ZB, and ZC, the impedance between the two terminating nodes equals: ZA+ZB+(ZAxc3x97ZB/ZC), as is well known in the relevant arts.
Thus, impedance ZC can be trimmed to obtain a desired impedance between the two terminating ends. In addition, a terminal of the switches used for trimming may be connected to a fixed voltage such that the resistance introduced by the switches remains substantially the same in many operating situations. Accordingly, the voltage between the terminating ends may remain substantially unaltered during operation.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.